When exactly would you use ".[^.]*" or ".??*" glob patterns?

Question

I'm trying to understand Linux, its command-line and this quote:

You can run into problems with globs because .* matches . and .. (the current and parent directories).  You may wish to use a pattern such as .[^.]* or .??* to get all dot files except the current and parent directories.

^{Source: How Linux Works: What Every Superuser Should Know by Brian Ward, 2nd Edition,
      Section 2.7 “Dot Files”, page 23 (Requires registration/login)
or   How Linux Works: What Every Superuser Should Know by Brian Ward, 3rd Edition,
      Section 2.7 “Dot Files”, page 22}

When exactly (in what command) would you use .[^.]* or .??*?

Answer 1

That's to work around a bug/misfeature in shells other than zsh, fish and the descendants of the Forsyth shell (including pdksh and derivatives)¹, whereby the expansion of the .* glob includes . and .. (on systems (most, unfortunately) where readdir() returns them)

With those shells,

chmod -R og-rwx .*

for instance would recursively remove rwx permissions to the current (.) and parent (..) directories instead of just the hidden files and directories in the current directory.

It's particularly bad for commands that do things recursively or act on directories like ls .*, chown -R .*, find .*, grep -r blah .* but it's still annoying for most other commands and I can't think of any commands for which you'd want to have those . and .. included in the list of files passed to them.

A safeguard had to be added to the rm utility to work around that misfeature as too many people were tripping on rm -rf .*.

With * added, it's also used to pass all files (hidden or not) as arguments to a command (cmd -- .[!.]* ..?* *), for which you'll find other workarounds depending on the shell.

The .[^.]* glob (.[!.]* in Bourne/POSIX shells) excludes . (as it matches on filenames with at least two characters) and .. (as the second character is . which doesn't match [^.]), but also excludes files like ..foo, for which you need the second glob ..?*.

Those . and .. are tools for directory traversal, it's a mistake that they should be listed like ordinary files. POSIX requires them to be understood in path components (like in open("."), stat("foo/../bar")) but not necessarily be implemented as directory entries nor included in readdir().

Still, most systems still do implement those like in the early Unices as hard links, and most of those that don't will still fake entries for them in the output of getdents()/readdir().

With bash, an alternative is to turn the dotglob option on and use:

chmod -R og-rwx [.]*

(though beware that if there's no non-hidden file, it could change the permissions of the [.]* file unless you had the failglob option on to mimic the behaviour of zsh/fish).

As a history note, filenames starting with . being hidden files were born from a coding mistake from someone trying to skip . and .. in the first place. It's ironical that when trying to do things with hidden files we would run into the same problem.

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^{¹ see also the globskipdots option in bash 5.2+}

Answer 2

When you'd like to move all files and directories with hidden names (starting with a dot) to another location:

mv .[^.]* old-dot-files/

Or, whenever you'd like to do anything on all dot-files or dot-directories in a directory:

for name in .[^.]*; do
   # process "$name"
done

Note that the pattern .??* requires the matched names to have at least three characters, so names like .a will not be picked up. .[^.]* on the other hand will skip anything with a double dot at the start of the name.

It may be better to explicitly test the matched names:

for name in .*; do
    # expecting a regular file
    if [ -f "$name" ]; then
        # process "$name"
    fi
done

for name in .*; do
    # expecting a directory other than . and ..
    if [ -d "$name" ] && [ "$name" != '.' ] && [ "$name != '..' ]; then
        # process "$name"
    fi
done

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In my experience, wanting to do something an all dot-file or dot-directories in a directory happens extremely seldom.